Abstract
Ligand-protected gold clusters with an asymmetric nature have emerged as a novel class of chiral compounds, but the origins of their chiroptical activities associated with helical charge movements in electronic transitions remain unexplored. Herein, we perform experimental and theoretical studies on the structures and chiroptical properties of Au13 clusters protected by mono- and di-phosphine ligands. Based on the experimental reevaluation of diphosphine-ligated Au13 clusters, we show that these surface ligands slightly twist the Au13 cores from a true icosahedron to generate intrinsic chirality in the gold frameworks. Theoretical investigation of a monophosphine-ligated cluster model reproduced the experimentally observed circular dichroism (CD) spectrum, indicating that such a torsional twist of the Au13 core, rather than the surrounding chiral environment by helically arranged diphosphine ligands, contributes to the appearance of the chiroptical response. We also show that the calculated CD signals are dependent on the degree of asymmetry (torsion angle between the two equatorial Au5 pentagons), and provide a visual understanding of the origin of helical charge movements with transition-moment and transition-density analyses. This work provides novel insights into the chiroptical activities of ligand-protected metal clusters with intrinsically chiral cores.
Highlights
Among the various metal clusters reported to date, the geometric structures of gold clusters protected by thiolate[1] and phosphine[2] ligands have been well-de ned through intensive Xray crystallographic investigations, promoting experimental studies into chiral gold clusters.[3]
As these circular dichroism (CD) responses were connected to the metal-based transitions at approximately 400–600 nm, it is quite reasonable to assume that torsion between the two Au5 rings of the Au13 cores contributes to the chiroptical activities of the Au13 clusters, we cannot ignore the effect of the surrounding chiral environments by the helically arranged achiral/chiral ligands on the CD responses.[4]
The intensity of the rst Cotton effect at 522 nm of the model (q 1⁄4 35) was +0.93 Â 10À40 esu[2] cm2 (Fig. 4a, black dotted line), which was approximately 1/350th of the theoretical intensity of that of 2-R.12. Such an enhancement effect on CD responses was theoretically demonstrated for several diphosphine-ligated gold clusters, where the chiral arrangement and/or nature of the ligands induced strong dissymmetric elds.5l,m these results clearly show that the torsion between the two Au5 rings of the icosahedral Au13 cores affects the CD responses of the Au13 clusters, and the helically arranged diphosphine ligands around the Au13 cores contribute to the signi cant enhancement of the responses
Summary
Among the various metal clusters reported to date, the geometric structures of gold clusters protected by thiolate[1] and phosphine[2] ligands have been well-de ned through intensive Xray crystallographic investigations, promoting experimental studies into chiral gold clusters.[3]. Based on a systematic approach using transitionmoment and transition-density analyses, we theoretically demonstrate a torsion-angle-dependent chiroptical response and provide a visual understanding of the origin of helical charge movements in the Au13 cluster.
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